It is well known that blood testosterone concentration decreases in male aging resulting in a decline in physiological functions. The rate-limiting step of testosterone biosynthesis is transfer of the substrate cholesterol to the mitochondrial inner membrane to begin the steroidogenic process. Steroidogenic acute regulatory (StAR) protein is critical for this cholesterol transfer. However, StAR protein has been shown to be significantly reduced in the aged Leydig cell and results in a decrease in mitochondrial cholesterol transfer. Recent studies suggested that cyclooxygenase-2 (COX2) produces a tonic inhibition of StAR gene expression. We have shown that COX2 increases in the aged Leydig cells and the increase in COX2 is associated with the decreases in StAR protein and blood testosterone. Intriguingly, the decreases in blood testosterone were reversed by feeding the aged rats a COX2 inhibitor. These studies suggest that a novel and previously uncharacterized mechanism is involved in testosterone decline during male aging. To study the molecular mechanism for the role of COX2 in testosterone biosynthesis in the course of aging and to evaluate approaches to reverse or delay the age-related decline in testosterone biosynthesis, the following Aims are proposed: 1) over expressing the COX2 gene in Leydig cells to study the effect of COX2 expression on StAR gene expression and steroid hormone production;2) to study the effects of the COX2 knock-out on luteinizing hormone-stimulated StAR gene expression and testosterone production in mice;3) to study the role of 5-lipoxygenase in the regulatory effect of COX2 on StAR gene expression and testosterone production in Leydig cells;4) to identify the arachidonic acid metabolites produced in the COX2 pathway that inhibit StAR gene expression and testosterone production;5) to determine the elements in StAR promoter DNA sequences responsive to the identified metabolites. The long-term objectives of this project are to understand the molecular mechanism for the role of COX2 in testosterone biosynthesis in the aging process and to explore the potential for new drug development or application of COX2 inhibitors to improve the health of the aged male.